I. Field of the Invention
The present invention relates to a method for densifying previously sintered parts of powdered metals, ceramics and the like.
II. Description of the Prior Art
In the liquid phase sintering of powdered metals, ceramics, and the like, the powdered material is first intermixed with a fugitive binder which holds the part in the desired shape after cold pressing. Usually this fugitive bonder or "wax" consists of a paraffin, polyetheleneglycol or a metal containing a hydrocarbon. The cold pressed part is conventionally known as a preform.
The preforms are then subjected to a presintering step in which the preforms are slowly heated thus vaporizing the fugitive binder and the vaporized binder is removed from the part by a wash gas, vacuum pumping or other means. Following the presintering step, the parts retain their shape despite the absence of the fugitive binder.
The parts are then subjected to a sintering operation in which the parts are raised to their liquid phase temperature which not only densifies the parts but also further releases any residual contaminants contained within the parts. These contaminants are removed from the part during the sintering operation by vacuum pumping or by flowing a wash gas, such as hydrogen, across the parts. Following the sintering of the parts, the parts are sufficiently dense and hard for many applications.
For applications requiring still further densification, greater strength of the sintered part or better internal integrity, these properties of the part can be improved by subjecting the part to hot isostatic pressing or "HIP" processing. During HIP processing, the parts are elevated to their liquid phase temperature and subjected to pressures in excess of 5,000 psi and typically in excess of 10,000 psi, for a period of 60 to 90 minutes. The primary advantage of HIP processing is to eliminate virtually all porosity within the part as well as greatly minimizing larger randomly spaced holes, slits or fractures which may be present in the part provided such holes, slits or fractures are not open to the surface.
A primary disadvantage of HIP processing is that, due to the high temperatures and high pressures used during the HIP processing, the previously known HIP equipment is extremely massive in construction and expensive to produce and acquire. Furthermore, the long cycle time for the HIP processing limits the production volume of HIP equipment and greatly increases the per part cost of the parts which are HIP treated.